This invention relates to marine drives, and more particularly to a drive incorporating the concept of maintaining the drive propeller in a position so that it pierces the surface of the water during normal operating conditions. Such so-called surfacing drives are often used in high speed competition, and it is important that they provide maximum output with minimum drag.
Generally, a marine surfacing drive can be defined as a drive wherein at least one blade of a propeller is disposed above the water surface at design conditions, i.e., high speed. Such a drive can also be defined as a drive in which the propeller centerline is generally adjacent or above the water surface, again at design conditions.
Examples of known marine surfacing drives are disclosed in U.S. Pat. Nos. 3,933,116, 4,544,362 and 4,565,532. The constructions disclosed in these patents incorporate a single propeller carried by a single propeller drive shaft. U.S. Pat. No. 4,544,362 additionally discloses an embodiment in FIG. 11 thereof of a pair of separate engines driving a pair of laterally spaced separate surface piercing propellers mounted on separate drive shafts. Other U.S. patents of general interest in this regard are U.S. Pat. Nos. 3,057,320 and 3,430,603, although the units described therein are apparently not intended for normal continuous driving in surfacing mode.
Most marine stern drives, to which the present invention is basically directed, include a propeller carrier from which downwardly depends a stabilizing fin or skeg just forwardly of the propeller itself. The skeg is designed to be parallel to the boat centerline during straight forward travel. See, for example, the member 90 in the aforementioned U.S. Pat. No. 4,544,362.
Problems have been observed with the known single propeller surfacing drives. Because the upper portion of the propeller is out of the water, the propeller creates a strong lateral force on the propeller carrier. To keep the boat on a straight forward course, this lateral force must be countered by steering the propeller carrier so that the skeg is turned so that it assumes an angular, rather than parallel, position with respect to the boat centerline. The result is the creation of undesirable high drag forces as the angular skeg is pushed through the water and propeller thrust is directed at an angle to the boat path.
Further, if the surface piercing propeller bounces in and out of the water due to wave conditions or trim attitude, radical and destabilizing steering forces can result. A large skeg may help to alleviate this problem, but again at the cost of high drag.
In addition, the imbalance in steering caused by the angularly adjusted skeg results in making the boat easy to turn in one direction, but very difficult to turn in the opposite direction.
Due to the aforesaid difficulties, many applications of high speed surfacing drives have required dual engines, dual propeller drive shafts, and laterally disposed dual oppositely rotating propellers, in an attempt to overcome the problems. However, the high cost and weight of dual systems has made them less than practical.
It is an object of the present invention to essentially eliminate the unbalanced lateral forces on the propeller carrier, even when the drive is used in a continuously surfacing mode, and without utilizing expensive and heavy dual drives, thus allowing propeller thrust to be aligned with the boat path.
Broadly in accordance with the aspects of the invention, means are provided to maintain a balance of lateral forces on the common carrier of contra-rotating surface piercing propellers when the skeg and propeller thrust are disposed parallel to the boat centerline and the drive is in surface piercing condition.
More specifically, a marine stern drive for a boat includes a propeller assembly having a carrier for a pair of concentric drive shafts to which are mounted a pair of closely adjacent fore and aft coaxial surface piercing propellers mounted on a common axis. The carrier also includes a downwardly extending skeg. The shafts are connected to a source of power and drive the propellers in contra-rotating relationship at essentially equal rotational velocities. The carrier is connected to devices for swinging the carrier laterally, for steering, and also vertically. A control is provided for positioning and maintaining the carrier vertically such that both contra-rotating propellers are continuously disposed in surface piercing position during normal operation of the drive. The result is that lateral forces created on the propeller carrier by one rotating surface piercing propeller are counterbalanced by the other propeller when the skeg and propeller line of thrust are parallel to the boat centerline.
Another aspect of the invention provides that the leading (or fore) edges of both propellers are relatively sharp for surface piercing, while the trailing (or aft) edges of both propellers are relatively blunt. The sharp leading edge of one propeller faces the blunt trailing edge of the other. While coaxial reverse-rotating propellers are known in marine drives, such as in U.S. Pat. Nos. 4,529,387 and 4,619,584, they have previously been fully submerged in normal operation and did not create undesirable lateral forces on the propeller carrier, as has been the case with propellers which are designed to normally pierce the water surface.